Stratospheric Aerosol Measurement Research Articles

Southern hemisphere stratospheric aerosol measurements: 2. Time variations and the 1974–1975 aerosol events

A balloon‐borne photoelectric counter and jet impactor have been used to examine the southern hemisphere stratospheric aerosol from late 1974 to mid‐1976. Changes in particle morphology and loading observed in this period are consistent with a new source of aerosol in 1974. In the 10‐ to 16‐km altitude range the loading of particles with r > 0.15 μm increased by slightly greater than a factor of 2 from September 1974 to July 1976, while the loading of particles with 0.028 μ m < r < 0.15 μm decreased by approximately a factor of 2. The increased loading of particles with r > 0.15 μm in the 1975 southern winter was only about one‐fifth that observed at northern mid‐latitudes in the preceding northern winter.

Stratospheric dust measurements, 1970–1977

Stratospheric aerosol measurements have been made by laser radar at São José dos Campos (23°S, 46°W) since July 1970. From 1970 to 1974 the measured scattering ratios for the 20‐km layer were similar to those reported for other locations. A large increase was observed in April 1975, 6 months after the eruption of Volcán de Fuego in Guatemala. This delay in the transport of the Fuego dust to our latitude is interpreted as being due to the inhibition of southward eddy transport during the southern summer by the mean meridional circulation. A maximum monthly average scattering ratio of 1.28 measured at a wavelength of 5890 Å was observed at 20 km in August 1975. Since early 1976 the 20‐km scattering ratio has oscillated around a value of about 1.15, but the integrated columnar backscattering coefficient above 17 km, indicative of the total stratospheric dust loading, has increased by about 40%. Scattering from the upper stratosphere, at heights above 30 km, was observed in the southern springs of 1971, 1972, and 1973. There appears to be some evidence that the dust responsible for this scattering was of extraterrestrial origin.

Southern hemisphere stratospheric aerosol measurements 1. Simultaneous impactor and in situ single‐particle (light scatter) detection

Stratospheric aerosol concentrations obtained on seven balloon soundings by in situ photoelectric counting of individual particles and jet impaction with later electron microscope analysis have been compared. The potential error sources in the two sizing techniques appear sufficient to explain the observed differences in concentration. This implies an underestimate in the impactor‐derived precollection radius (for sulfuric acid aerosol) of about 25% relative to the photoelectrically determined radius in the two ranges r > 0.15 μm and r > 0.25 μm. The variance in impactor‐derived column loadings is determined by comparison of the impactor and photoelectric detector loadings; over the 10‐ to 28‐km altitude range, for example, this gives an error estimate of ≈ ± 18% for a 95% confidence level. An estimate of the flight‐to‐flight accuracy of the impactor sizing models is also determined from the loading comparison.

Combined use of lidar and numerical diffusion models to estimate the quantity and dispersion of volcanic eruption clouds in the stratosphere: Vulcán Fuego, 1974, and Augustine, 1976

Methods are described for estimating the quantity and dispersion on a global scale of the material injected into the stratosphere by a violent volcanic eruption by using a combination of lidar measurements of stratospheric aerosols and a previously described two-dimensional atmospheric dispersion model with the initial volcanic cloud as the ‘source function’ (Cadle et al., 1976). The technique involves comparing the lidar data with model predictions normalized by using data of various kinds obtained after the Gunung Agung eruption in Bali in 1963. In this paper we show that the estimates for the Fuego eruption of 1974 can be made equally well by comparing model and lidar measurements of peak stratospheric concentrations or the total masses within a range of altitudes in a column above the place where the lidar measurements were made. The lidar data previously reported have been extended to several months following the eruption of Augustine volcano in Alaska during January and February 1976. The results indicate that the Augustine eruption must have injected no more than one fiftieth of the amount of material into the stratosphere that was injected by Agung and confirm the previous finding (Cadle et al., 1976) that Fuego injected one fifth of the amount injected by Agung.

Measurements of stratospheric aerosols by the solar occultation method.

Measurements of stratospheric aerosols by the solar occultation method.

Measurements of stratospheric aerosols by airborne laser radar

More than 15,000 stratospheric backscattering profiles were measured with an airborne laser radar. This instrument was flown aboard the NASA Convair 990 jet aircraft on four flights over the Pacific Ocean south of Hawaii in August 1971 and on two flights between Wallops Island, Virginia, and Bermuda in September 1971. The laser echoes measured at 585-nm wavelength were 1.0–1.1 times the echoes expected from a dust-free molecular atmosphere at altitudes of 15–35 km with an uncertainty of ±0.1 at the 95% level of confidence. There were no well-defined scattering layers in that altitude interval for either of the experimental areas. These scattering ratios are significantly lower than those obtained by the same measurement technique in earlier years.

Stratospheric Aerosol Measurements with Implications for Global Climate

In this paper we present measurement results obtained in New Mexico with bistatic optical probing of the atmosphere using a searchlight beam. The data yield vertical profiles of the aerosol attenuation coefficient. Because they approximate proportionality to aerosol concentration, these profiles provide information concerning the aerosol layer structure and its parameters. During a 9-day period in October and November 1970, a series of forty-one such profiles was obtained which includes altitudes 12-25 km, selected for study because of the relatively high aerosol content of this stratospheric region and its relation to global climate. The mean stratospheric aerosol distribution for this period is double layered with maxima at 15.6 km and 19.3 km. An early phase of volcanic dust incursion is examined. A chronology of stratospheric aerosol concentration levels is developed based on measurements with the same instrumentation at the same sites since February 1963. The chronology shows that the particulate background level of a nonvolcanic, nonpolluted stratosphere is represented by an aerosol optical thickness Tau(p)(O.55 micro) = 2.0 x 10(-2). A calculation with this background level is included, which indicates feasibility of a laser-satellite method for acquiring data related to global climate. The Appendix introduces an original method of numerical integration (used in the calculations), which is suited to radiative transfer studies.

Stratospheric aerosol measurements by optical radar

A laser radar is being used at Adelaide, South Australia (35°S) for routine studies of upper atmosphere densities and aerosol concentrations to an altitude of 60 km. The transmitter operates at a wavelength of 0.694 μ and generates pulses of 0.2 J energy and 0.5 μsec pulse length at a rate of 20 per min. The receiver is a 12 in. Newtonian telescope equipped with an interference filter of bandwidth 8 Å. The light is detected by a cooled photomultiplier, and the recording is carried out by integrating photon counts from selected height intervals. Results for the period February–June, 1969 show that, over the height range 10–60 km, the scattering can be considered as molecular with the exception of the region 13–24 km where enhanced scattering is attributed to aerosols. The maximum aerosol number density, averaged over the five months, is estimated as 1.3 × 10 6 m −3 and occurs at a height of 18.5 km.